In pattern test apparatuses, test light is required to be of sufficient amount to test pattern defects in a test sample such as a photomask, in which a minute pattern is formed. Thus, a method of improving the lighting efficiency by a combination of a quarter-wave plate and a polarizing beam splitter (PBS) is used, as a coaxial episcopic illumination method.
Specifically, S-polarized light from the light source is reflected by the PBS, converted into circularly polarized light by a quarter-wave plate, and applied to the surface of the mask. Then, the light reflected from the surface of the mask is converted into P-polarized light by the quarter-wave plate, and made incident on the PBS. The light which has transmitted through the PBS is detected by a sensor.
In the above method, however, when the pattern formed on the photomask has a periodic structure which is shorter than the wavelength of the test light, the polarized state of the light reflected from the surface of the mask is changed by the influence of form birefringence, in which the refractive index differs according to the orientation of the electric field of the incident light. Thus, the light reflected from the surface of the mask cannot be efficiently transmitted through the PBS. Therefore, a reduction in the amount of light for pattern testing is inevitable, leading to reduced test accuracy.